Sputter deposition is a generally known method of depositing a layer of material onto the surface of a substrate workpiece, such as a semiconductor wafer. In sputter deposition, a substrate to be coated is placed generally opposite a sputter target of the material to be deposited and an ionized gas plasma is formed in the area of the target. The target is electrically energized with a negative electrical polarity opposite the polarity of the ionized gas particles in the plasma, while the deposition chamber or enclosure is grounded so as to create an electric field at the target. The ionized plasma atoms under the influence of the electric field, bombard the target surface and dislodge or "sputter" particles of target material which travel away from the target. Since the substrate is usually positioned opposite the target, the sputtered particles deposit on the substrate to form a deposition film layer of the target material on the substrate.
Reactive sputter deposition is a form of sputter deposition which occurs in the presence of a chemically reactive gas, such as nitrogen or oxygen, in addition to an inert gas, such as argon, which is used to form the ionized plasma. The plasma produces the ions that sputter the target ejecting particles that project onto the substrate. As the target particles are deposited on the substrate, they come into contact and react with the reactive gas at the surface of the substrate causing a film to form on the substrate surface that is a product of the chemical reaction. Therefore, in reactive sputter deposition, the composition of the deposited film is a chemical combination of both the target material and the reactive gas. For example, sputtering an aluminum target in the presence of inert argon gas will yield an aluminum film on the substrate. Alternatively, when particles are reactively sputtered from an aluminum target in the presence of an argon-oxygen gas combination, the film deposited on the substrate is the ceramic aluminum oxide. Similarly, targets sputtered in the presence of other reactive gas combinations will yield various other reactant films on the substrate.
A recurring problem with reactive sputtering is that the chemical reaction between the reactive gas and the target particles takes place not only at the surface of the substrate but also at the sputtering surface of the target. As a result, an undesired reactant film forms on the target. This problem is particularly troublesome when the reactant film that is formed is an electrically insulating material, such as, for example, the ceramic aluminum oxide. When an insulating reactant film is formed on the surface of the target, it causes plasma instabilities, electrical arcing, and the generation of contaminating particles in the sputter deposition chamber. These results are undesirable in a sputter deposition chamber and adversely affect the proper deposition of a film on a substrate.
It has been proposed to provide a primarily inert gas such as argon in the area of the target and to provide a primarily reactive gas such as oxygen in the area of the substrate. While this method may be effective to prevent reactant layers from forming on the target, the method requires that the distance between the sputter target and the substrate be increased in order to reduce the tendency of the inert gas and the reactive gas to intermix and in order to physically isolate the target from the reactive gas. When the distance is increased, however, the deposition rate is reduced and the thickness uniformity of the film deposited on the surface of the substrate is also reduced. Uniform thickness of the deposition film is often a desirable quality in both reactive and non-reactive sputter deposition processes.
Another way of reducing the formation of a reactant film on the surface of the target, is to bias the target such that it is sputtered away at a rapid rate that is faster than the reaction rate of the reactive gas with the target. Rapidly sputtering the target yields a clean target surface and prevents the formation of a reactant layer on the target. However, when the rate of sputtering is increased, the rate of deposition on the substrate is also increased. At an increased deposition rate, the sputter particles do not have time to sufficiently react with the reactive gas at the surface of the substrate, and the desired reactant film is not formed. It has also been proposed to rapidly sputter the target and increase the concentration of reactive gas at the substrate surface. However, an increased concentration of gas requires the introduction and evacuation of gas at a rate that may not be possible without the use of an increased number of pumps or larger pumps, both of which result in a greater processing expense and complexity. Further, certain properties of the deposited film such as the resistivity of the film and its morphology or crystalline nature are adversely affected by a rapid deposition rate despite the presence of higher concentrations of reactive gas.
Another drawback in sputter deposition is the non-uniformity of the film that is deposited upon the substrate. The uniformity of the film thickness is a function of many factors, including the relative ion concentration and the shape of the gas plasma which sputters the target, such as that which results from the shape of the magnetic fields which may be used to enhance the plasma formation over the target. Other factors include the shape of the vacuum chamber containing the substrate, the spatial effects of any other structures or devices located within the chamber proximate the target and the substrate, as well as any external or internal electric or magnetic field effects associated with sputter deposition chamber. The shape of the target and its orientation with respect to the substrate may also result in a non-uniform film.
It is, therefore, an objective of the present invention to reduce the formation of a reactant film on the surface of the sputter target while providing the proper reaction conditions at the substrate surface for the deposition of the desired reactive material film on the substrate. It is a further objective of the present invention to provide a means for selectively controlling the rate of deposition on a substrate and selectively varying the thickness of a deposited film, such as to improve the uniformity of the thickness of a sputter deposited film.